The new work explores the link between such reward-seeking and the study of social interactions, said the lead author of the report Galit Shohat-Ophir, now of the Howard Hughes Medical Institute in Virginia, US.

"It is thought that reward systems evolved to reinforce behaviours that are important for the survival of both individuals and species, like food consumption and mating," Dr Shohat-Ophir told BBC News.

"Drugs of abuse kind of hijack the same neural pathways used by natural rewards, so we wanted to use alcohol - which is an extreme example of a compound that can affect the reward system - to get into the mechanism of what makes social interaction rewarding for animals."

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'Control system'

Working in the laboratory of Ulrike Heberlein at the University of California, San Francisco, Dr Shohat-Ophir and colleagues subjected a number of flies to a wide variety of fates.

In one set of experiments, male flies were put in a box with five virgin females, which were receptive to the males' advances. In another, males were locked up with females that had already mated and which thus roundly rejected the males' attempts at sex.

Offered either their normal food slurry or a version charged with 15% alcohol, the mated males avoided the alcohol, whereas the sexually deprived males went on a comparative bender.

The team then went on a hunt for a chemical that could tie the two parts of this story together, hitting on neuropeptide F (NPF).

They found that the heavy-drinking rejected males had a lowered level of the chemical, and sated, mated males had an elevated level.

"What we think is that these NPF levels are some kind of 'molecular signature' to the experience," Dr Shohat-Ophir explained.

To show that the NPF is actually responsible for the change rather than just associated with it, the researchers actively manipulated just how much NPF was in the flies' brains.

Those with depressed levels acted like the rejected males, and those with elevated levels behaved like the mated males.

"What this leads us to think is that the fly brain - and presumably also other animals' and human brains - have some kind of a system to control their level of internal reward, that once the internal reward level is down-regulated it will be followed by behaviour that will restore it back," Dr Shohat-Ophir said.

It is tempting, given that humans share a similar brain chemical, to imagine that NPF drives human behaviour as well.

However, in an accompanying article in Science, Troy Zars of the University of Missouri wrote that "anthropomorphising the results from flies is difficult to suppress, but the relevance to human behaviour is obviously not yet established".

Nevertheless, he suggested that the work linked "a rewarding social interaction with a lasting change in behaviour".

"Identifying the NPF system as critical in this linkage offers exciting prospects for determining the molecular and genetic mechanisms of reward and could potentially influence our understanding of the mechanisms of drugs of abuse."

ABSTRACT

The brain’s reward systems reinforce behaviors required for species survival, including sex, food consumption, and social interaction. Drugs of abuse co-opt these neural pathways, which can lead to addiction. Here, we used Drosophila melanogaster to investigate the relationship between natural and drug rewards. In males, mating increased, whereas sexual deprivation reduced, neuropeptide F (NPF) levels. Activation or inhibition of the NPF system in turn reduced or enhanced ethanol preference. These results thus link sexual experience, NPF system activity, and ethanol consumption. Artificial activation of NPF neurons was in itself rewarding and precluded the ability of ethanol to act as a reward. We propose that activity of the NPF–NPF receptor axis represents the state of the fly reward system and modifies behavior accordingly.